Activists claim the Sats ruin children’s education as schools are put under pressure to “teach to the test” to boost results. It is also claimed that failure to hit targets puts head teachers’ jobs under threat.

Up to 600,000 children in England’s 17,000 state primary schools were due to sit a reading test on Monday and separate writing and spelling exams on Tuesday. Pupils will take a series of maths tests on Wednesday and Thursday.

The boycott is set to prevent the publication of exam league tables later this year and will also throw Ofsted inspections into chaos as the watchdog relies on exam data as a key barometer of school standards.

Christine Blower, NUT general secretary, said that up to half of primaries were set to join the action.

"We have reports that the numbers of schools boycotting Sats in many areas is close to 50 per cent,” she said. “At that level of participation in the action, it would be impossible to draw up league tables."

Ahead of the boycott, the NUT held an "anti-Sats picnic" in central London on Sunday, which was billed as a celebration of reading. Unions also claim the exams prevent children reading whole books.

Alan Gibbons, the children’s author, who was due to attend, said: “Schools use extracts to spot the metaphor or the simile, instead of allowing children to read whole books.

“We have seen a real increase in the technical dismantling of literature with the specific aim of hitting targets and doing well in exams.”

A survey of almost half of England’s 150 local authorities suggests schools in almost every area will boycott exams.

All 31 schools in Hartlepool will be affected and an estimated 75 per cent of primaries in Calderdale, West Yorkshire, will refuse to stage Sats.

As many as half of primaries in Kent – England’s biggest authority – are likely to boycott tests, while in Barnsley, South Yorkshire, more than half will take part.

Some 60 schools in Manchester are backing the action, while 32 in Hertfordshire and 16 in Barking and Dagenham said they were boycotting.

In total, 37 local authorities confirmed schools would be taking part in the action, 21 did not yet have figures and only five said no primaries were boycotting.

The action was expected to be the first battle a new government would have to face, but with a hung Parliament following Thursday's General Election, it is highly unlikely there will now be a last-ditch attempt by politicians to get it called off.